This invention relates to motors, and more specifically to high performance linear motors.
Linear motors are commonly used, for example, in micro-lithographic instruments for positioning objects such as stages, and in other precision motion devices. A linear motor uses electromagnetic force (normally called Lorentz force) to propel a moving part.
In FIG. 1A (reproduced from FIG. 1 of Itagaki et al. U.S. Pat. No. 4,758,750, incorporated herein by reference in its entirety) a conventional linear motor includes magnets 2 which form one magnet pair and create a magnetic field in between. The magnetic poles N (north) and S (south) are shown. Similarly, the adjacent magnets form another magnet pair and create a magnetic field of opposite polarity. The width of two adjacent magnets plus two spaces between the magnets defines the magnetic pitch PM of the motor. The magnetic flux direction across a gap 4 is indicated by arrows 7 and 7a. A moving coil unit 12 has electrically conductive wires laid out in a direction perpendicular to the plane of the figure. An electric current is passed through the wires, in a direction either into the plane of the figure or out of the plane of the figure.
As those skilled in the art will recognize, a wire carrying an electric current in a magnetic field creates Lorentz force, the formula of which is:
F=NLBxc3x97I
Where F represents Lorentz force, N the number of wires, B the magnetic flux, and I the electric current. For a coil with a given length L and magnetic flux B, to maximize force F, one has to maximize the number of wires N and current I. The above formula determines both the magnitude and the direction of force F, since force F, magnetic flux B, and current I are all represented as vectors, and the symbol xe2x80x9cxc3x97xe2x80x9d represents vector cross product multiplication. As those skilled in the art will recognize, a task in motor design is to maximize F/{square root over (P)}, or the xe2x80x9cmotor constantxe2x80x9d where
F/{square root over (P)}=NLBI/({square root over (I2R+L )})=NLB/{square root over (R)}.
In the above expression, F is the scalar value of vector F, while P is the amount of power consumed by the motor. For each particular design configuration, the motor constant is directly related to the xe2x80x9ccopper density,xe2x80x9d which is defined as the total wire cross sectional area as a percentage of the entire coil cross section. (The coil wires are often made of copper.)
In the configuration shown in FIG. 1A, the Lorentz force created by the current in coil unit 12 causes the coil to move. While traveling in the right direction of FIG. 1A, coil unit 12 eventually leaves the field of magnets 2 and enters the field of the adjacent magnets. Since this second magnetic field has a reversed polarity relative to that of the first magnetic field, the current in coil unit 12 must reverse in polarity so as to maintain the direction of Lorentz force. The reversal of the direction of the electric current is accomplished by a commutation circuit familiar in the art (not shown).
FIG. 1B, reproduced from FIG. 2 of Itagaki et al., is a cross-sectional view of the conventional linear motor of FIG. 1A, viewed along the line IIxe2x80x94II in FIG. 1A. In such a linear motor at the coil head area 12xe2x80x2, the coil heads are stacked on top of each other. This arrangement requires a wide head area 12xe2x80x2.
Such a conventional linear motor has several disadvantages, one of the which is the difficulty of installation and removal. As shown in FIGS. 1A, 1B, a magnetic track is formed by magnets 2 and the magnetic side rails 3. The magnetic track has a wide head area configured to match the shape and size of the wide head area 12xe2x80x2 of coil assembly 12. To remove coil assembly 12 from the magnetic track, coil assembly 12 must slide away from the magnetic track in a direction perpendicular to the surface of the paper. Since the equipment (e.g. an X-Y stage) attached to coil assembly 12 is often heavy and difficult to handle, special tools are typically required during installation and removal of coil assembly 12.
Another disadvantage of a conventional linear motor coil is its low efficiency. FIG. 2 shows a cross sectional view of a linear motor coil taken at a cross section perpendicular to the wire direction. Since the wire is not close packed, air gaps 50 inevitably result, substantially lowering the conductor density of the coil. As discussed above, lower conductor density often corresponds to lower motor efficiency.
It is therefore desirable to provide a linear motor having a motor coil with improved efficiency, low heat dissipation, and easy installation.
A motor in accordance with the invention overcomes the above and other drawbacks of conventional linear motors. According to the invention, a linear motor comprises a motor coil in cooperation with an associated magnetic track. The motor coil includes a linear assembly of coil units, each similar to the other. Each coil unit has an electrically conductive wire wound into a closed band in a predetermined number of layers, typically a single layer. The shape of the closed band is geometric polygonal, such as diamond shaped, hexagonal, or double diamond shaped, having inner edges surrounding a void. Some embodiments comprise a row of parallelogram shaped closed bands folded into a row of double diamond shaped coil units. In some embodiments, the width of the void is an integral multiple of the width of the closed band.
The coil units are made e.g. of flex circuit material or by winding electrically conductive wires in a racetrack or folded tip fashion. In some embodiments the width of a coil unit is equal to the magnetic pitch of the associated magnetic track. In other embodiments the width of a coil unit is equal respectively to one-half or two-thirds of the magnetic pitch.
Advantageously this arrangement provides high electrical efficiency and ease of disassembly. The coil units are stacked together in a partially overlapped fashion to form a row of coil units in the motor coil so that the number of layers of wire in the useful area is substantially uniform across the entire coil. Unlike the wide end coil shape of Itagaki et al., the present shape is more planar (not flared out at the end) and so has a flat cross section that allows the coil to be easily removed from and installed in the magnetic track.